As is known, knitted articles, during their production on machines, in particular on circular knitting machines, must be constantly subjected to adequate takedown in order to allow a correct and regular formation of the stitches.
In order to achieve this, adapted takedown devices are used which can be constituted by simple pneumatic suction devices, such as for example in circular machines of small diameter, or by more complex mechanical devices composed of one or more takedown rollers, on which the article is wrapped at least partially and which can generate greater takedown forces, for medium- and large-diameter circular machines.
Such takedown devices are essentially required to apply to the article being formed a substantially constant takedown force. This requirement can be met by way of two different operations, termed respectively “constant-torque operation” and “constant-speed operation”. The more suitable operation type is selected according to the type of knitting to be produced. If there are no variations in the characteristics or structures of the knitting during the production cycle, “constant-speed operation” can be employed. If instead the production cycle provides for variations of the type or quantity of the knitting produced, such as for example in machines with electronic selection, with particular reference to those that transfer the knitting from needles to contiguous needles, the more suitable operation is constant-torque operation.
In constant-speed operation, a transmission ratio is set between the needle cylinder of the machine that produces the article and the takedown roller with which the article engages. In this manner, each variation of the rotation rate of the needle cylinder is matched by a variation of the rotation rate of the takedown roller. In order to achieve this result, reduction units and speed variators are used to transmit the rotary motion of the needle cylinder to the takedown roller. Alternatively, the takedown roller can also be driven by an independent electric motor that is driven with a rotation rate which is correlated to the rotation rate of the needle cylinder according to a preset ratio.
In constant-torque operation, the rotation of the takedown roller can undergo speed variations depending on the type of knitting being produced so as to keep the torque applied to the takedown roller and thus the takedown force applied to the article being formed substantially constant.
Takedown devices with constant-torque operation of the known type use, for driving the takedown roller, a torque limiter or a spring reloading system or an electric motor which can deliver a constant torque as the rotation rate varies.
Among devices with constant-torque operation, the takedown devices with spring reloading are, as a whole, the simplest devices to provide and manage. Such devices generally comprise at least one takedown roller, which is arranged with its axis in a horizontal position and is supported, so that it can rotate about its own axis, by a supporting structure arranged below the needle cylinder of the circular knitting machine. The takedown roller generally cooperates with other rollers so as to achieve the partial wrapping of the article on its lateral surface in order to have the adhesion to the article that is needed to actuate its takedown. Such devices are provided with a motor element, which can rotate with a reciprocating motion, along an arc of rotation, about a main axis, which generally coincides with the takedown roller axis. The motor element is connected to the takedown roller via a freewheel mechanism, which connects the motor element to the takedown roller only in one direction of rotation, whereas in the opposite direction of rotation it sets the motor element free with respect to the takedown roller. The motor element is connected to a spring that is connected to the supporting structure and whose function is to turn the motor element in the direction of rotation in which it is connected to the takedown roller. Moreover, the motor element is provided with an arm which engages portions of face cams, which are fixed to the supporting structure of the machine with respect to which the needle cylinder can be activated with a rotary motion around its vertically arranged axis.
In practice, in these devices, during the production of the knitted to article, the needle cylinder rotates about its own axis with respect to the supporting structure. The supporting structure of the takedown device rotates together with the needle cylinder and therefore the arm of the motor element engages progressively the portions of the face cams that generate the rotation of the motor element about the main axis in the direction of rotation in which the motor element rotates freely with respect to the takedown roller. This rotation, which has no effect on the takedown roller, produces however the loading of the spring, which, when the engagement of the motor element arm with the face cam portions ends, causes the rotation of the motor element about the main axis in the opposite direction, i.e., in the direction of rotation in which the motor element is rotationally connected to the takedown roller. The torque produced by the spring is thus transmitted to the takedown roller, which therefore applies a corresponding takedown force to the article that is engaged with it. By adequately positioning the spring and providing a large number of alternating rotations of the motor element, of reduced extent, at each turn of the needle cylinder about its own axis, it is possible to have a substantially constant torque that is produced by the spring and transmitted to the takedown roller.
The presence of a number of portions of face cams, which are necessary to have an adequate number of alternating rotations of the motor element, constitutes a structural complication of the machine and therefore one tends to limit as much as possible the number of said face cam portions, with the consequence, however, of having intolerable variations of the torque and therefore of the takedown force.
Moreover, in these devices there is no possibility of varying the extent of the rotation of the takedown roller as the quantity of knitting produced by the machine varies. In practice, these devices are designed as a function of the maximum theoretical quantity that can be produced by the machine and therefore do not allow to achieve high quality in the production of the articles.